It is often desirable to create a cavity or undercut in an inaccessible portion of a substrate, such as a back face or between the opposite faces of the substrate, the undercut having dimensions larger than the hole drilled into the substrate. Such undercuts are especially useful to attach fastening devices to brittle or low strength substrates because they allow the use of expanding anchoring devices which can spread the load over a greater area of the substrate, thereby lowering the stress and increasing the pull-out strength of the fastener.
Bone matter is one example of a substrate in which expanding anchors may be used to good effect. When used in bone matter, conventional fasteners, such as screws, fail at relatively low loads because the screw distributes the load only to a small area of the substrate immediately around the screw, resulting in high stresses in the substrate. Fastener loads in bone matter are also limited by the composition of the bone matter, which comprises a relatively hard cortical layer surrounding an inner cancellous layer of relatively soft tissue. The cancellous layer is largely ineffective at retaining the fastener, thus concentrating the load from the fastener entirely in the hard cortical tissue and raising the stress even further.
To deploy an expanding anchor in bone matter, it is advantageous to create an undercut in the cancellous layer behind the cortical layer, the undercut having a larger diameter than the hole drilled through the cortical layer and thereby forming a shoulder on the inaccessible back face of the cortical layer. The anchor is inserted through the drilled hole, allowed to expand within the undercut and engage the shoulder to spread the load over a relatively large area of the back face of the cortical layer, resulting in reduced stress and increased fastener pullout loads.
Existing apparatus for creating undercuts in back faces or beneath substrate surfaces tend to be specialized tools which are inserted into a pre-drilled pilot hole to a predetermined depth. A cutting blade is then deployed from the tool and the tool is rotated to carve out the undercut and form the shoulder beneath the surface of the substrate. Such tools tend to be complicated and require an extra step in that a pilot hole must first be drilled into the substrate and then the tool employed to carve out the undercut in the otherwise inaccessible interior or back face. There is clearly a need for a simpler tool which combines both the operation of drilling the pilot hole and creating the undercut beneath the surface of the substrate.